The functionality of lambda probes at combustion engines as a part of the exhaust gas purification system has to be checked within the range of the on-board-diagnosis (OBD). This also includes the checking of manipulations at lambda probes, which influence the probe dynamic. For detecting a reduced lambda probe dynamic it is nowadays usual to evaluate a reaction of the probe upon a lambda change in the engine control systems. Therefore a step-wise lambda change is for example used such as during the transition from a rich to a lean exhaust gas mixture at a boost turn off. A step-wise lambda course would result in an approximately step-wise course of the probe signal at an erroneous lambda probe. Deviations from this course, for example a delay or a slurring of the probe signal indicate on the other hand a reduced probe dynamic. In the praxis however a step-wise course of the air value lambda for example downstream of a catalytic converter is generally not given. Moreover does the lambda course already provide an unknown more or less slurred profile, which has an effect upon the reaction of the probe. For this reason those check procedures can only detect comparably long delays in the probe course, which lie for example in the range of 100 ms. Shorter delays can on the other hand not clearly be used as an indication for a reduced probe dynamic.
A sufficiently good probe dynamic is now a significant requirement for an optimal purification of the exhaust gas. It is therefore the task of the invention to detect electric manipulations at any random position in the signal circuit between the sensor element of the lambda probe and for example an analog digital converter (ADC) in the control unit, which cause a delay or slurring of the probe signal. Significantly shorter delay times should thereby be reliably detected than with the previous methods.